Insulated gate bipolar transistors, short IGBTs, have been established as semiconductor power switches, i.e., for switching high currents at high voltages. Typical applications include switched circuits such as in power converter systems, for example, switched power mode supplies, short SPMSs. In these applications an IGBT typically is switched on and off in fast succession. Accordingly the switching characteristics are of interest.
In one embodiment IGBTs are used in switched mode power supplies, wherein two IGBTs are arranged in a half-bridge configuration, i.e., a high-side IGBT is coupled with its collector to the anode of a direct voltage source, the high-side IGBT's emitter is connected to the collector of a low-side IGBT, and the low-side IGBT's emitter is coupled to reference ground. By alternately turning the IGBTs on and off the coupling point between the IGBTs can be switched to the voltage of the direct current source or to reference ground. The resulting voltage at that coupling point can be smoothed by using a smoothing capacitor.
Basically an IGBT can be switched from its non-conducting state, i.e., the so-called off-state, to its conducting on-state by applying a positive gate-emitter voltage higher than the threshold voltage, i.e., in this way an IGBT can be turned on. To turn an IGBT off, i.e., to switch an IGBT from its on-state to its off-state, a zero voltage or a negative base-emitter voltage can be applied to an IGBT.
The IGBTs can be controlled using pulse width modulated signals of opposite phase, so the high-side IGBT is switched to a conducting state, i.e., switched to its on-state, while the low-side IGBT is switched to its off-state and vice versa. Due to the alternate switching, the voltage at the connection point switches between reference ground and the voltage of the voltage source.
For providing control voltages to drive the high-side IGBT, i.e., for providing a gate-emitter voltage to the high-side IGBT, a bootstrap circuit can be used, which provides voltages referenced to the connection point of the half-bridge arrangement of the IGBTs. However, when using unipolar gate driving a parasitic current oscillation during the turn-on transient of the IGBT, an unintended parasitic turn-on can be caused by high voltage-rise across the IGBT switch due to the Miller effect. To remove the parasitic current oscillation, a negative gate voltage is required at the high-side IGBT. To prevent an IGBT from unintended parasitic turn-on caused by the Miller effect, a Miller clamp can be provided. However, these solutions have drawbacks. In order to provide a negative gate voltage an SMPS needs to have an increased power rating and additional circuitry, the SMPS thus getting more complex and costly. Also the Miller clamp causes additional circuitry in the boot-strap circuit thus further increasing complexity and costs.
Hence an improved boot-strap circuit is needed that allows to remedy the above problems while using as few components as possible.